Silicon-based solar cells Characteristics and production processes ...

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Silicon-based solar cells – characteristics and production processes The method which allows for a further purification of the mc-Si silicon and the obtaining of mono-crystalline silicon is zone melting, in which the FZ (Float Zone) silicon is obtained. The basic method of the Si mono-crystal production for the electronic industry is the Czochralski method. It involves the crystallization of the melted Si on the rotating mono-crystal nucleus, and the formation of a big monocrystal in the form of a roller, with the crystallographic orientation defined by that of the nucleus. The silicon for the PV sector is also manufactured through the stretching of Si from the melted material out to the form of a ribbon (R-Si), which is created on a graphite matrix (the EFG (Edge-defined Film-fed Growth) silicon) or between strained strings (the SR (String Ribbon) silicon). This method is used in practice by the American company, Ever Green. Similar results can be obtained through the method of deposition on molybdenum or graphite bases (the RGS (Ribbon Growth on Substrate) silicon) or ceramic bases (the SOC (Silicon on Ceramic) silicon) [11]. There is a number of other methods of crystalline silicon production for photovoltaics, such as the recrystallization of Si powder by means of laser or the crystallization of amorphous silicon through the reaction with the aluminium layer, but basically, those reckoned in the mass production of solar cells based on Si type p are: the polycrystalline mc-Si silicon manufactured through directional crystallization, in the process of casting, in blocks of a typical weight of 450 kg; the mono-crystalline Cz-Si silicon, manufactured with the Czochralski method; and, partially, the ribbon R-Si silicon. In the last range, one can observe a certain increase in the interest in the Cz-Si silicon type n, which gives the possibility to produce solar cells with the E ff value above 24 % [12]. The increase in the degree of silicon utilization is also stimulated by the improvement in the process of cutting blocks into single plates, with the use of diamond wire saws. Due to the technology development, the presently applied 200 micrometer wire diameters are being replaced by 150 – 120 µm ones, and this directly effects the increase in the number of the plates obtained from one block of crystalline silicon. The technological advancement in the production processes of the photovoltaic sector requires large financial inputs. An exemplary cost of an SG mc-Si silicon factory, with the capacity of about 3000 tons per year, allowing for the production of solar cells of the power of 275 MW p per year, constructed by the Wacker Schott Solar company, in Jena, Germany, will amount to over 300 mln Euro. The mc-Si silicon factory, with the annual production capacity of 8000 tones, constructed in Taiwan by Formosa Chemicals & Fibre Corporation, will cost 1,03 billion USD [13]. The world’s biggest silicon supplier, Hemlock, is planning an investment of the order of 4 billion USD, which, in 2014, will make it possible to increase the production from the current 30000 tons to 63000 tons per year [14]. 13
Silicon-based solar cells – characteristics and production processes 2. Crystalline silicon cells 2.1 Photovoltaic effect – solar cell’s current and voltage The photovoltaic effect, which is the physical basis for the change of electromagnetic energy into electric one, consists in the creation of a noncompensated space electric charge in the given material medium, as a result of radiation absorption. The presence of this non-compensated charge results in the creation of electromotor force equaling the difference of potentials present at the terminals of the unloaded cell. If the cell terminals are closed with an outer circuit, then the conductor will be filled with direct current, its intensity being dependent on the value of the external resistance. In order for the cell to generate electric current, it is necessary to modify its structure by separating the positive and negative charge carriers in the conduction band. This separation occurs as a result of the carrier diffusion between the areas of different carrier concentrations, according to the electrochemical potential gradient, and also as a result of the charge convection in the internal electric field of the cell. An example of such a medium is the semiconductor crystalline silicon with a p-n junction. The formation of an area with the p-type conductivity takes place as a result of doping with atoms of acceptor elements for Si, from group III of the periodic system, whereas an area with the n- type conductivity is created as a result of doping with atoms of donor elements for Si, from group V of the periodic table. If the above structure of the crystalline Si material is exposed to radiation whose quantum energy is higher than its energy gap E g = 1,12 eV, then a result of the light absorption is the generation of electric charge carrier pairs, electron-hole, which become separated under the effect of the electric field present in the junction. The consequence is an excess of electrons on the n-side and an excess of holes on the p-side, which results in the formation of electric voltage [15]. In a real solar cell made on the basis of crystalline silicon type p, the band structure, the potential distribution and the intensity of the electric field in the space charge of the p-n junction depend on the concentration of the donor impurity N E and the acceptor impurity N A . Figure 6 presents the band structure for an illuminated and non-illuminated solar cell, depending on the impurity concentration. The N A impurity of the base material type p, homogeneous throughout the material, usually has the value of 1,5x10 16 atom/cm 3 , which corresponds to the resistivity value of 1 Ωcm and it is usually obtained by boron doping. The N E value is dependent on the impurity’s profile and the concentration of the donor impurity N D in the surficial area. Figure 6 schematically shows the effect of the formation of photovoltage V as a result of the cell’s illumination. The calculations performed with the use of a PC-1D computer 14
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